f simplify somewhat

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -53,7 +53,7 @@ use util::config::UserConfig;
 use util::events::{EventHandler, EventsProvider, MessageSendEvent, MessageSendEventsProvider, ClosureReason};
 use util::{byte_utils, events};
 use util::scid_utils::fake_scid;
-use util::ser::{BigSize, FixedLengthReader, Readable, ReadableArgs, MaybeReadable, Writeable, Writer};
+use util::ser::{BigSize, FixedLengthReader, Readable, ReadableArgs, MaybeReadable, Writeable, Writer, VecWriter};
 use util::logger::{Level, Logger};
 use util::errors::APIError;
 
@@ -3716,14 +3716,14 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 	fn get_htlc_temp_fail_err_and_data(&self, desired_err_code: u16, chan: &Channel<Signer>) -> (u16, Vec<u8>) {
 		debug_assert_eq!(desired_err_code & 0x1000, 0x1000);
 		if let Ok(upd) = self.get_channel_update_for_unicast(chan) {
-			let enc = if desired_err_code == 0x1000 | 20 {
-				let mut res = Vec::new();
+			let mut enc = VecWriter(Vec::with_capacity(upd.serialized_length() + 4));
+			if desired_err_code == 0x1000 | 20 {
 				// TODO: underspecified, follow https://github.com/lightning/bolts/issues/791
-				res.extend_from_slice(&byte_utils::be16_to_array(0));
-				res.extend_from_slice(&upd.encode_with_len());
-				res
-			} else { upd.encode_with_len() };
-			(desired_err_code, enc)
+				0u16.write(&mut enc).expect("Writes cannot fail");
+			}
+			(upd.serialized_length() as u16).write(&mut enc).expect("Writes cannot fail");
+			upd.write(&mut enc).expect("Writes cannot fail");
+			(desired_err_code, enc.0)
 		} else {
 			// If we fail to get a unicast channel_update, it implies we don't yet have an SCID,
 			// which means we really shouldn't have gotten a payment to be forwarded over this